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- Collision Dynamics of a Single Droplet onto a Heated Dry Surface: Jet Fuel and HVO MixturesPublication . Pinto, Pedro Miguel Moreira; Vasconcelos, Daniel; Ribeiro, Daniela; Barata, Jorge M M; Silva, AndréUnderstanding the phenomena behind droplet impact is essential to improve the effectivenessin many industrial applications, such as fuel injection in an internal combustion engine andrapid spray cooling of hot surfaces. This study focuses on an experimental investigation ondroplet impact onto a heated wall. The purpose of this work is to analyse the influence of walltemperature on the morphology of a single droplet impact and observe the possible outcomes.In these experiments, the observation of all heat regimes was possible. Since there is an urgeto implement biofuels in general aviation, the fluids analysed are fuel based. The fluids usedwere distilled water, as a control group, 100% jet fuel, and a mixture of 50% biofuel with 50%jet fuel. This mixture corresponds to the maximum of renewable energy source fuel due to theminimum of 50% of jet fuel required by civil aviation. The impact energy was kept constant, andthe Weber number was set to 320. Furthermore, different wall temperatures were chosen (25◦C−320◦C) to seek every possible impact phenomenon and characterise the impact morphology.The impact dynamics were captured using a high-speed digital camera and the images weredigitally processed.
- Influence of wetting behavior on the morphology of droplet impacts onto dry smooth surfacesPublication . Foltyn, Patrick; Ribeiro, Daniela; Silva, André; Lamanna, Grazia; Weigand, BernhardThe influence of wettability on the morphology of droplet impacts onto dry surfaces is often neglected in the literature, despite its significant effect on the resulting morphology. In this work, the role of wettability is investigated systematically by considering droplet impact processes on smooth dry surfaces of two different materials. The wetting behavior is varied not only by employing two different fluids, but most importantly by varying the surface properties by plasma activation and polymerization. Overall, this leads to four different wetting behaviors for each surface. The changes in impact morphology are visualized by means of a three-perspective experimental facility. In particular, the bottom view employs a total internal reflection-configuration for visualizing the exact droplet contact area and contact time. This enables us to characterize the main features of the different wetting behaviors. Overall, we found that surface wettability mainly influences the receding phase, resulting in higher receding rates with decreasing wettability but also the maximum spreading diameter.
- Measurement of the lamella thickness during droplet impact onto differently wettable smooth surfaces using an extension of the LASER Pattern Shift Method with naturally occurring patternsPublication . Foltyn, Patrick; Rihm, Lynn Kristin; Ribeiro, Daniela; Silva, André; Weigand, BernhardThis study shows that the LASER Pattern Shift Method (LPSM) is a powerful measurement technique for film thickness measurements. In this paper, the approach of the LPSM is extended, which is now able to measure the lamella thickness during droplet impacts on smooth surfaces using the naturally occurring air bubble shadows. With the help of this rather new measurement technique, the influence of different experimental parameters on the lamella thickness could be systematically assessed, e.g., the influence of impact velocity, liquid properties, and surface wettability. Upon comparing the obtained results to an analytical correlation in the literature, good agreement could be found for its validity range.
- 3D Simulations of Droplets Impacting Liquid Films: Crown Parameters MeasurementsPublication . Vasconcelos, Daniel; Ribeiro, Daniela; Silva, R. D.; Barata, Jorge M MThe 3D incompressible Navier-Stokes equations are coupled with the CLSVOF method and employed to numerically simulate the phenomena of single droplet impact onto liquid films. A solution-adaptive mesh refinement tool, based on the gradient of the volume fraction scalar, is adopted in order to reduce computational cost. Three different fluids are taken into account: 100% jet fuel and 75%/25% and 50%/50% of jet fuel and biofuel, respectively. Quantitative analysis of the crown height and outer diameter is performed for different impact conditions, such as the influence of the impact velocity and dimensionless thickness, between experimental and numerical results, and the qualitative analysis includes the occurrence of splashing and overall crown evolution. Numerical results show that the crown outer diameter measurements are in good agreement with the experimental cases, presenting a slight discrepancy for the lower liquid film thickness of h*= 0.2. The crown height measurements are under-predicted for the current model, maintaining a similar trend for dimensionless thicknesses of h*= 0.5 and h*= 1 while, for the lower thickness, the crown disintegrates at earlier stages. The crown curvature and rim instabilities exhibit significant differences, and the splashing phenomenon occurs for both the experimental and numerical outcomes.
- Insights on Bubbling Formation after Drop Impact on Thin Liquid FilmsPublication . Ribeiro, Daniela; Panão, Miguel; Silva, A. R. R.; Barata, Jorge M MOver the years, the phenomena obtained when a drop impinges upon a dry, wetted or heated surface have been thoroughly studied. In previous works, the existence of splash was investigated by the authors with the goal of evaluating the possible implementation of biofuels in the civil aviation and it was found an episode of a phenomenon, seldom reported in the literature under specific pre-impingement conditions. The mechanism that leads to a bubble formation has two stages. After the drop impacts a steady liquid film, prompt splash occurs followed by crown splash. In the first stages of crown splash, the uprising sheet propagates almost normal to the liquid film, but its radius at the base continues to expand, eventually leading to the inward collapse of the crown bounding rim. Thus, the top of the crown closes in a bubble-like shape with the formation of two jets, one upwards and other downwards. The upward jet eventually disappears due to gravitational influence, while the downward jet continues to grow until it reaches the liquid film, attaching to it, stretching and detaching from the top at the hemispheric thin-sheet, forming a perfect bubble. Many secondary droplets fall on the bubble and one of them will eventually break the dome, leading to more secondary atomization. The few works reported in the literature referring to this phenomenon as “bubbling” or “floating bubble,” scarcely explore the hydrodynamic mechanism associated with this bubble formation and occurrence, mainly focusing on droplets impacting upon deep pools. However, in a previous study, the authors observed this event for a liquid film dimensionless thickness of 0:5 in a fluid mixture of Jet A-1 and biofuel NEXBTL. In this study, the impact conditions in the experiments performed allow to recreate the floating bubble with 100% of occurrence. After that, the authors present an extensive characterization of the bubbling phenomenon to understand better the mechanisms which lead to its formation, as well as its practical significance. A high-speed digital camera acquires several images of the floating bubble formation from different points of view (side and bottom). Namely, capturing the phenomenon from below, high-quality images allow retrieving essential data to describe the hydrodynamic mechanism accurately. The most relevant features include the bubble height and diameter, and the propagation velocity of the first perturbation imposed on the liquid film.
- Aerodynamic Breakup of a Single Droplet due to a Crossflowed AirstreamPublication . Carrolo, Gabriel Alexandre da Costa; Ribeiro, Daniela; Barata, Jorge M M; Silva, A. R. R.The main breakup regimes are visualized and studied with the help of a high-speed camera and a continuous jet of air, provided by a wind tunnel, is used to transition the droplet into the various stages of secondary atomization. The use of Jet Fuel and Biofuel mixtures is the focal point of this study. All the known regimes are encountered and each transitional We is calculated. The variation of We due to Oh is described, in addition to the comparison between We number thresholds and correlations existent in the literature for each regime transition. The influence of these numbers on the dimensionless characterization of time and later the maximum cross stream diameter is shown. Finally, a correlation for the temporal evolution of the drops’ cross stream diameter for all regimes, assuming its deformation is not a linear function of time, is proposed.
- Does liquid film temperature affects single drop impact dynamics?Publication . Mendes, André F. S. F.; Vasconcelos, Daniel; Ribeiro, Daniela; Panão, Miguel; Silva, A. R. R.The effect of liquid film dynamics in the hydrodynamics of an isolated drop impact is a complex phenomenon and not fully understood. Therefore, in this work, an experimental setup built to characterize the impact of an isolated droplet on heated and unheated liquid films consists of a heating element made of an aluminum block with resistances to produce several impact conditions. The parametric studies include the drop impact velocity and size for different fluids to evaluate their properties effect on the phenomena. The results were compared with existing thresholds in the literature to evaluate their validity and applicability range. This comparison allows us to assess if temperature causes the limits of the thresholds to change drastically or if its influence is negligible. Regarding IC engines, thresholds like splashing and bubble encapsulation are significant since they influence the atomization of the mixture and, consequently, the pollutant emissions.
- Experimental Study of a Single Droplet Impinging upon Liquid Films: Jet Fuel and Biofuel MixturesPublication . Ribeiro, Daniela Filipa Santo; Silva, André Resende Rodrigues daThe present work is focused on a single droplet impinging upon a liquid film of the same fluid. This particular study is a matter of interest for several research areas and has a wide variety of applications such as fuel injection in internal combustion engines and processes involving spray paints, coatings and systems cooling. The human being started searching for new alternatives to reduce pollution, and since transports contribute with a significant portion, it is extremely necessary to bet on alternatives to fossil fuels. The introduction of biofuels in the aviation sector could be an example. The huge challenge is to modify and optimize piston engines to operate efficiently with alternative fuels. In order to achieve that, in these experiments, Jet Fuel and Biofuel mixtures were used. The main goal of this dissertation is to visualize the dynamic behavior of single droplets impinging upon liquid films with different relative thicknesses, several outcomes are possible. To accomplish that, four fluids were used: water (as reference), 100% Jet A-1, 75%/25% and 50%/50% mixtures of Jet A-1 and NEXBTL, respectively, since civil aviation only accept mixtures with at least 50% Jet Fuel in volume. To assure the accuracy of the calculations, the fluids physical properties were measured. An experimental facility was designed and built, and the setup includes a high-speed digital camera that was manually triggered with a specific exposure time. The impact site was illuminated by a led lamp through a diffusion glass to provide uniform back lighting. A syringe pump connected to the needle released the droplets with a specific pumping rate. The liquid film is held by a perspex container. Five needles were used with different inner diameters to yield five distinct droplet sizes for each fluid. Additionally, three impact heights were established to provide three impact velocities and Weber numbers for each needle. The liquid films depths considered were 10%, 50% and 100% of the droplet diameter. The existence of splash was reported as well as its characteristics. Some conclusions about the influence of the impact conditions and the fluids physical properties were indicated. Using the obtained data comparisons were made with some splashing thresholds available in the literature.
- Dynamic behaviour of single droplets impinging upon liquid films with variable thickness: Jet a-1 and hvo mixturesPublication . Ribeiro, Daniela; Cunha, Nuno; Barata, Jorge M M; Silva, AndréFortunately, the human being has already started to be environmentally concerned and the search for new alternatives to reduce pollution increased. Transports are responsible for a significant portion and it is extremely necessary to bet on alternatives to oil. The introduction of biofuels in aero-engines could be an example. In order to modify and optimize piston engines and gas turbines to operate efficiently with alternative fuels, this work used Jet Fuel and Biofuel mixtures. The focus of these studies was to visualize the dynamic behavior of single droplets impinging upon liquid films with variable thickness. The existence of splash as well as its characteristics were reported and the differences and similarities between the outcomes according to the impact conditions and the fluid properties were catalogued. To achieve that an experimental facility was designed and built. Four fluids were tested: water (as reference), 100% Jet A-1, 75%/25% and 50%/50% mixtures of Jet A-1 and HVO (Hydro-processed Vegetable Oil), respectively, since civil aviation only accept mixtures with at least 50% Jet Fuel in volume. The fluid properties were measured to ensure accuracy. The liquid film depths considered were 10%, 50% and 100% of the droplet diameter. A high-speed digital camera was used to image acquisition and the droplet was released by a syringe pump connected to the needle at a specific pumping rate. The impact surface was a perspex container filled with fluid. Five needles with different inner diameters and three impact heights were employed to provide a variety of Weber and Reynolds numbers.
- The influence of Wettability on the Droplet Impact onto Micro-Structured SurfacesPublication . Ribeiro, Daniela; Foltyn, Patrick; Silva, André; Lamanna, Grazia; Weigand, BernhardThe flourishing of applications in need of self-cleaning mechanisms increased the search forwater repellent hydrophobic surfaces with induced roughness. Disclosing the small-scale inter-face phenomena on the wetting behavior is essential to design efficient hydrophobic materialswith defined topography. On the other hand, the spreading behavior concerning the forma-tion of thin films on a surface is required to assure the quality of spray cooling and coatings.The contact angle undoubtedly plays an important role in the droplet impact, providing differ-ent outcomes. Moreover, an open question is, how surface topography can affect the impactprocess. Therefore, to evaluate these matters, different surface patterns were manufacturedto assess the surface topography influence on the impact dynamic behavior. Additionally, thewettability of the micro-structured surfaces was flexibly influenced through plasma activationand plasma polymerization. The impact of distilled water and isopropanol droplets on the dif-ferent surface patterns was captured from three perspectives providing high-quality images ofthe phenomena. Different surface morphologies can be obtained depending on the surfacemicro-structures and wettability, affecting spreading shape and evolution. The fluid penetrationwithin the micro-structures is a key feature influencing not only the structures of the outcomesbut also the transition between regimes.
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